1. Field of the Invention
This application relates to a process for the preparation of a compound of the formula I ##STR3## in which each Hal independently represents a chlorine or bromine atom and A represents a hydrogen or metal atom or an alkyl group of 1 to 4 carbon atoms.
2. Description of the Prior Art
There exists a great deal of interest in the preparation of the above cyclopropane compounds, since they are intermediates in the preparation of certain synthetic pyrethroid insecticides. These specific pyrethroids are esters which consist of an acid portion and an alcohol portion, wherein the acid portion is derived from a 3-(2,2-dihalovinyl)-2,2-dimethylcyclopropane carboxylic acid (cf. formula II).
Conventionally, the desired cyclopropane ring is formed by a base-catalyzed cyclization of a suitably substituted compound related to hexanoic acid. In the most general of terms, such reactions can be represented as follows: ##STR4## Here Hal is halogen and either X is halogen and Y is hydrogen or X and Y together form an olefinic double bond. In the prior art, A and B can represent a variety of possible groups such that the desired acid (or an ester thereof) can be prepared. Bases employed are both bases in the Bronsted sense, e.g. KOH, and bases in the Lewis sense, e.g. organic amines. Examples of documents in which such reactions are disclosed are German Offenlegungsschrift No. 2606635 and UK specifications Nos. 1561502, 1512419, 1520023 and 1580203.
In general these reactions proceed smoothly and in good yield. However, the cyclization reaction has to be carried out in the presence of a base, which necessitates working up the salts or complexes formed initially in the reaction mixture. In some cases this work-up may be quite complicated due to the formation of salts or complexes, which are difficult to separate. Consequently there is a need for an alternative process which obviates these problems, and which process is generally applicable to the cyclization of cyclopropane compounds too.
Suprisingly it has now been found that the cyclization can be performed in the presence of an acid.